The natural world around us abounds in colors. Manmade things, too, are full of colors. Manufacturers of these manmade things use pigments in the manufacturing process to add colors. Pigments are powders that, when mixed with a liquid, make a paint or ink. Black iron oxide is an inorganic pigment that is used extensively in many industries, such as the coating, asphalt, construction, food, cosmetics, data recording and plastics industries. Black iron oxide is shipped to manufacturers all across the country in large quantities, often in the thousands of pounds.
Black iron oxide contains iron molecules in a +2 oxidation state. The iron in such an oxidation state is thermodynamically unstable and will oxidize when exposed to an oxidizing environment, such as air. When oxidized, black iron oxide turns reddish brown, what many people recognize as a rust color, and is expressed by the chemical equation 4Fe3O4+O2 6Fe2O3+Heat.
It is well known in the pigments industry that oxidation of black iron oxide is an exothermic reaction and liberates heat, a process known as self-heating. During self-heating, spontaneous combustion of black iron oxide is common.
The United States Department of Transportation and the United Nations use a particular testing procedure to classify self-heating substances for transport identification purposes. In the test, a dry sample of a substance to be tested is placed in a sample container. The sample container containing the dry sample is then placed in an oven and subjected to an oven temperature of approximately 140 degrees celcius. The dry sample temperature is monitored continuously over a 24 hour period by use of a thermocouple placed at the center of the dry sample and another thermocouple placed between the sample container and the oven wall. Monitoring thus, provides a measure of the temperature rise, if any, of the dry sample relative to the oven temperature. An approximate 60 degree celcius rise in the dry sample temperature as compared to the oven temperature indicates problematic self-heating that can cause spontaneous combustion of the dry sample, such as a sample of black iron oxide.
The problem of black iron oxides tending toward spontaneous combustion is complicated when the pigment is packaged in large quantities, particularly during storage and transportation. To prevent such spontaneous combustion of black iron oxide, it has become necessary to package the pigment in small quantities using many small containers, such as thick paper sacks to name one example, to accommodate large orders. Such packaging requires that a United Nations certificate be affixed thereto in order to comply with transport regulations. Because such packaging increases the cost of shipping, a better solution to prevent spontaneous combustion of black iron oxide is needed.
Attempts have been made at coating the black iron oxide sufficiently enough to shield the black iron oxide from an oxidizing environment, such as air, and thereby minimize self-heating. Information relevant to attempts to address these problems can be found in U.S. Pat. Nos. 4,382,982, 4,491,619, 5,039,553, 5,188,898, 5,449,565, 5,650,194, 5,714,299, 6,179,907 and 3,869,298. However, each one of these references suffers minimally from one or more of the following disadvantages: 1) use of Boron as a coating, 2) use of silicates as a coating and 3) use of non-aluminum coating.
For the foregoing reasons there is a need in the industry for a simple means to coat black iron oxide sufficiently enough to shield the black iron oxide from an oxidizing environment and thereby improve the heat stability of black iron oxide. Such a solution to the problems described would eliminate the need for the use of United Nations certification and facilitate the storage and transportation to manufacturers of large quantities of black iron oxide pigments for coloring the manmade things that humans use.